The big solar disk blurs shadows
![[April Holladay] A solar eclipse shows the shadow that the Sun’s disk forms.](2002-03-13-shadows.jpg) Q: Does the slight blurring of shadows from sunlight on a clear day have more to do with
the size of the sun or with the atmosphere dispersing the sunlight? --Pascal Golay, Brooklyn,
NY
A: The size of the Sun determines the slight blurring of shadow edges. It's big disk gives multiple
light sources, says Sergei Smirnov, chemistry professor at New Mexico State University. The atmosphere scattering sunlight mainly
causes shadows on Earth to be lighter than they would be in a vacuum.
[April Holladay] A solar eclipse shows the shadow that the Sun's disk forms.
An eclipse of the Moon shows how the Sun's size blurs shadow edges. As the Moon glides between the Sun and us, she first casts a
weak shadow (called the penumbra and shown light gray in the figure) over part of daytime Earth, then she casts the blackest, deepest
shadow (black umbra) over a lucky few that see the Sun totally obscured. Finally Luna blocks the other side of the Sun and the Moon's
shadow lightens again as the penumbra slides by once more.
The light gray penumbra is the "blurring" of the Moon's shadow. If you're in the penumbra and look at the Sun (through safety goggles),
you'll see only part of the Sun's disk. Part of the Sun shines on you and part doesn't. Consequently, you end up in a light shadow.
Every object in the full Sun-casts such a shadow. The blurring of the shadow is the penumbra cast by the Sun's disk. In everyday life,
we call the umbra the "shadow" and many don't even notice the penumbra. But it's there and our readers notice it.
Earth's atmosphere scatters sunlight so light floods in from all over the sky. That's why a shadow on Earth is never totally dark even
though the object blocks the Sun's light. Whereas, when astronauts wandered the Moon, they could scarcely see anything in the dark
shadows there. Luna has no air to scatter sunlight.
(Answered by April Holladay, science correspondent, Mar. 13, 2002)
Further Surfing:
Conceptual Physics by Paul G. Hewitt
www.schorsh.com: An object's shadow
The Exploratorium: why eclipses happen
|
